Structure and dynamics of dinuclear Zirconium(IV) complexes

We have determined by X-ray crystallography the structures of three dinuclear zirconium(IV) complexes containing the heptadentate ligand dhpta (where H(5)dhpta = 1,3-diamino-2-propanol-N,N,N',N'-tetraacetic acid, 1) and different countercations: K-2[Zr-2(dhpta)(2)](.)5H(2)O (2(.)5H(2)O), Na-2[Zr-2(dhpta)(2)](.)7H(2)O(.)C(2)H(5)OH (3(.)7H(2)O(.)C(2)H(5)OH), and Cs-2[Zr-2(dhpta)(2)](.)H(5)O(2)(.)Cl(.)4H(2)O (4(.)H(5)O(2)(.)Cl(.)4H(2)O). In the K(I) complex 2, crystallized from water, the two Zr(IV) ions are 3.5973(4) Angstrom apart and bridged via two alkoxo groups (average Zr-O 2.165 Angstrom). Each Zr(IV) is eight-coordinate and also bound to two N atoms (average Zr-N 2.448 Angstrom), and four carboxylate O atoms (average Zr-O 2.148 Angstrom). The two dhpta ligands in the dinuclear unit have different conformations. One face of the complex contains an array of 14 oxygen atoms and interacts strongly with the two K(I) ions, one of which is 6-coordinate, the other 8-coordinate, which are 3.922(4) Angstrom apart and bridged by a carboxylate O and by two water molecules. The structures of the dinuclear anion [Zr-2(dhpta)(2)](2-) in the Na(I) complex 3 and in the Cs(I) complex 4 are essentially identical to that found in complex 2, although the alkali metal ions coordinate differently to the oxygen-rich face. All Zr(IV) ions have a distorted triangulated dodecahedral geometry. Although the crystal structure of complex 2 does not indicate the presence of acidic protons, in 4 an [H5O2](+) unit is strongly H-bonded to an oxygen atom of a coordinated carboxylate group. 1D and 2D H-1 and C-13 NMR spectroscopic and potentiometric studies reveal two deprotonations with pK(a) values of 9.0 and 10.0. At low pH, two carboxylate groups appear to undergo protonation accompanied by chelate ring-opening, and the complex exhibits dynamic fluxional behavior in which the two magnetically nonequivalent dhpta ligands exchange at a rate of 11 s(-1) at pH 3.30, 298 K, as determined from 2D EXSY NMR studies. Ligand interchange is not observed at high pH (>11). The same crystals of complex 2 were obtained from solutions at pH 3 or 12. The dynamic configurational change is therefore mediated by the aqueous solvent.